JP-A-2006-201129 corresponding to US Patent Application Publication No. 2006-164254 proposes a vehicular seating detection sensor using a double-electrode capacitance sensor having an electrode assembly in which a front electrode is provided on the front major surface of a resin film buried in a seat, and a back electrode which is smaller than the front electrode is provided on the back major surface of the resin film. The double-electrode capacitance sensor also has a detection circuit section for detecting whether someone is seated or not using the electrode assembly.
The detection circuit section applies an AC voltage between one electrode and the vehicle body and supplies an AC current to the other electrode via an operational amplifier so that the potential of the other electrode becomes equal to that of the one electrode. The detection circuit section judges whether someone is seated or not on the basis of a variation of the AC current.
Although the above double-electrode capacitance sensor is higher in detection sensitivity than single-electrode capacitance sensors, there is a market requirement that its detection sensitivity be increased further.